Genetic and regulatory factors influencing the expression of human A, B, H, Lea, Leb, I and i blood group antigens on cell surface are being analyzed. Cultured human epithelial cells which may express A, B, or H antigens at their surface are fused to HPRT-deficient mouse cells to produce hybrids. Human mouse hybrid cells positive for specific blood group surface antigens are selected by affinity to lectin-coated nylon fibers. Progeny of these selected positive cells will be karyotyped to determine the chromosome(s) carrying the blood group loci.